As a rule, in conventional common-control telephone systems with scanning of the loop conditions of the subscriber circuits centralized scanning means periodically checks all subscriber circuits and, if necessary, also junctors consecutively one after the other for comparing their momentary statements with the previous statement so as to determine any possible changes in the subscriber circuit or junctor statement and to initiate appropriate new switching processes. In these known scanning processes, particularly for the purpose of determining if a subscriber set wishes to establish an outgoing connection, previous connection of a subscriber circuit to the junctor circuit is not necessary. Therefore that is also assumed that the loop is buffered in a subscriber line circuit and it also implies the use of at least one or two relays or indicators for each subscriber line circuit.
When using electronic crosspoints, e.g. transistors, thyristors, etc., as crosspoints, special measures would have to be taken for the automatic ring disconnection, if subscriber line circuits are employed for recognizing and storing of the loop statements, since no direct-current loop supervision can occur over such switching elements. The ring disconnection in prior art systems is supervised and controlled from the subscriber line circuit, which of course, results in a greater outlay for each subscriber line circuit.